1. Field of the Disclosure
The present disclosure is in the field of gamma ray testing of geological formations. In particular, the disclosure determines the organic carbon content of a formation from recorded spectra.
2. Description of the Related Art
Well logging systems have been utilized in hydrocarbon exploration for many years. Such systems provide data for use by geologists and petroleum engineers in making many determinations pertinent to hydrocarbon exploration and production. In particular, these systems provide data for subsurface structural mapping, defining the lithology of subsurface formations, identifying hydrocarbon-productive zones, and interpreting reservoir characteristics and contents. Many types of well logging systems exist which measure different formation parameters such as conductivity, travel time of acoustic waves within the formation and the like.
One class of systems seeks to measure incidence of nuclear particles on the well logging tool from the formation for purposes well known in the art. These systems take various forms, including those measuring natural gamma rays from the formation. Still other systems measure gamma rays in the formation caused by bursts of neutrons into the formation by a neutron source carried by the tool and pulsed at a preselected interval.
In these nuclear well logging systems, reliance is made upon the physical phenomenon that the energies of gamma rays given off by nuclei resulting from natural radioactive decay or induced nuclear radiation are indicative of the presence of certain elements within the formation. In other words, formation elements will react in predictable ways, for example, when high-energy neutrons on the order of 14.2 MeV collide with the nuclei of the formation elements. Different elements in the formation may thus be identified from characteristic gamma ray energy levels released as a result of this neutron bombardment. Thus, the number of gamma rays at each energy level will be functionally related to the quantity of each element present in the formation, such as the element carbon, which is present in hydrocarbons. The presence of gamma rays at a 2.2 MeV energy level may for example, indicate the presence of hydrogen, whereas predominance of gamma rays having energy levels of 4.43 and 6.13 MeV, for example, may indicate the presence of carbon and oxygen respectively.
In these nuclear well logging systems, it is frequently useful to obtain data regarding the time spectral distributions of the occurrence of the gamma rays. Such data can yield extremely valuable information about the formation, such as identification of lithologies that are potentially-hydrocarbon producing. Moreover, these desired spectral data may not only be limited to that of natural gamma rays, for example, but also may be desired for the gamma ray spectra caused by bombardment of the formation with the aforementioned pulsed neutron sources.
Well logging systems for measuring neutron absorption in a formation use a pulsed neutron source providing bursts of very fast, high-energy neutrons. Pulsing the neutron source permits the measurement of the macroscopic thermal neutron absorption capture cross-section Σ of a formation. The capture cross-section of a reservoir rock is indicative of the porosity, formation water salinity, and the quantity and type of hydrocarbons contained in the pore spaces.
The measurement of neutron population decay rate is made cyclically. The neutron source is pulsed for 20-40 microseconds to create a neutron population. Neutrons leaving the pulsed source interact with the surrounding environment and are slowed down. In a well logging environment, collisions between the neutrons and the surrounding fluid and formation atoms act to slow these neutrons. Such collisions may impart sufficient energy to these atoms to leave them in an excited state, from which after a short time gamma rays are emitted as the atom returns to a stable state. Such emitted gamma rays are labeled inelastic gamma rays. As the neutrons are slowed to the thermal state, they may be captured by atoms in the surrounding matter. Atoms capturing such neutrons are also caused to be in an excited state, and after a short time gamma rays are emitted as the atom returns to a stable state. Gamma rays emitted due to this neutron capture reaction are labeled capture gamma rays. In wireline well logging operations, as the neutron source is pulsed and the measurements made, the subsurface well logging instrument is continuously pulled up through the borehole. This makes it possible to evaluate formation characteristics over a range of depths.
Depending on the material composition of the earth formations proximal to the instrument, the thermal neutrons can be absorbed, or “captured”, at various rates by certain types of atomic nuclei in the earth formations. When one of these atomic nuclei captures a thermal neutron, it emits a gamma ray, which is referred to as a “capture gamma ray”.
Prior art methods exist for determining attributes of a formation from logging results. See, for example, U.S. Pat. No. 4,712,424, to Herron, U.S. Pat. No. 4,394,574, to Grau et al., U.S. Pat. No. 4,390,783, to Grau, SPE 7430 of Hertzog, SPE9461 by Westaway et al., and U.S. Pat. No. 5,471,057, to Herron.
In the exploration for and production of hydrocarbons, it is essential to analyze the geological basin involved. In determining the hydrocarbon generation potential of an area, source rocks (i.e. any rock capable of producing hydrocarbons) must be identified, along with volume of the rock and the quantities of organic matter contained therein. Identification of the presence of source rock is usually critical in deciding whether to continue drilling a well or to abandon it. U.S. Pat. No. 4,071,755 to Supernaw discloses a method in which the energy spectrum of natural gamma radiation occurring in earth formations penetrated by a well borehole is observed in energy regions corresponding to uranium, potassium and thorium. Quantitative evaluations of the relative abundances of these elements are made by comparing the observed spectra with standard gamma ray spectra. The relative abundances of these elements may then be interpreted in terms of the organic carbon content of earth formations by comparison with predetermined relationships found to exist therebetween. U.S. Pat. No. 4,686,364 to Herron et al. discloses a method for determining in situ the carbon content of a source rock comprises determining the carbon/oxygen elemental ratio of the formation via inelastic gamma spectroscopy, determining the porosity of the formation, obtaining the oxygen contents and densities of the fluid and minerals in said formation, and determining the carbon content from said carbon/oxygen ratio, and said oxygen contents and densities of the fluid and minerals. The present disclosure is directed towards a method of determining the organic content of a formation using only nuclear instruments.